//#include <stdc++.h>
#include <bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    double a, b = 0;
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> a;
        b += 1.0 / a;
    }
    b /= n * 1.0;
    printf("%.2f", 1.0 / b);
    return 0;
}
